The present disclosure relates generally to transferring images in image forming devices. More particularly, the present disclosure describes an apparatus, method, and system useful for transferring an image from an image bearing belt to a web material.
Image transfer systems that use photoconductive belts as the image bearing surface for building the image can use non pressure transfer nips employing corona devices for transferring the image to the substrate. A problem exists when trying to print on rough web substrates like webs with labels or perforations. Air breakdown at the transfer nip can cause image disturbance around label edges and perforations. Air breakdown refers to ionization of air caused by air within a field where the field exceeds the Paschen curve for air as described by Paschen's Law. The ions created by this ionization cause many different types of image disturbances as they react with the many charged particles and surfaces involved with electrophotography.
A pressure transfer system can be used to eliminate the air in the transfer nip which will significantly reduce break down and image disturbance. A transfer assist blade can be used, but does not provide enough force in the transfer nip to significantly eliminate the air gaps around heavier substrates with perforations and like what is encountered in label webs. Rolls that are electrically biased, called biased transfer rolls, can be used to provide transfer nip pressure and electric field for image transfer in systems that do not use photoconductive belts to significantly reduce the air in the transfer nip thus significantly reducing any image disturbances.
A problem with using a biased transfer roll with a photoconductive belt is that photoconductive belts can have a grounded layer which can cause a field between the biased transfer roll and the photoconductive belt in the area just before the transfer nip. This field can cause pre-nip breakdown and image disturbance which can range from graininess to heavy mottle, white spots, and large area deletions.
Using a biased transfer roll pressure nip may cause motion quality issues at the point of the photoconductive belt seam or possible arc through if the seam is not sufficiently insulated from the ground layer.